The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to methods for improving the toughness of zonal isolation materials.
Set cements that are tough are particularly desirable in the context of well cementing. In general, a well that is more than a few hundreds of meters deep is cased, and the annular space between the underground formation and the casing is cemented over all or part of its depth. The essential function of cementing is to prevent fluid exchange between the different formation layers through which the borehole passes and to control the entry of fluids into the well, in particular to limit the entry of water and gas. In production zones, the casing, the cement and the formation are all perforated, typically by the use of explosive perforating charges, over a few meters.
The cement positioned in the annular space in a well is subjected to a number of stresses throughout the well's lifetime. The pressure inside the casing can increase or decrease as the nature of the fluids therein changes, or when additional pressure is applied within—for example during a stimulation operation. Such pressure changes may cause the casing to expand or contract, thereby exerting stress on the cement sheath.
Temperature changes also exert stress on the casing, which in turn affects the cement sheath. Such temperature changes may arise owing to cement hydration and the pumping of fluids into the well whose temperatures are significantly different from those in the wellbore.
Mechanical shocks may be exerted by perforating operations. Perforating may not only causes an overpressure of several hundred bars inside the well, but the energy also dissipates in the form of a shock wave. Perforating also disturbs the cement when the charge penetrates the cement sheath and subjects the zone surrounding the borehole to large forces extending several meters into the formation.
Another process that creates dynamic stresses in the cement sheath is when a window is cut through a cemented casing to create a sidetrack. Milling the steel over a depth of several meters followed by drilling a sidetrack subjects the cement to shock and vibration, which may cause irreversible damage.
Over the course of a well's productive life, and after abandonment, seismic events may disturb the borehole and the cement sheath, potentially disrupting the cement sheath and causing the loss of zonal isolation.
The well cementing industry has responded to these challenges by developing cement systems with improved flexibility. Such cement systems may have lower densities than conventional cements, or they may contain fibers, flexible particles or both. They generally have lower Young's moduli than conventional cements. A review of these systems is presented, for example, in the following publication: Nelson E B, Drochon B, Michaux M and Griffin T J: “Special Cement Systems,” in Nelson E B and Guillot D (eds): Well Cementing—2nd Edition, Schlumberger, Houston (2006) 233-268.
Despite the valuable contributions offered by these technologies, it would be desirable to have even more durable cement systems thereby ensuring that the cement sheath maintains zonal isolation during and after hydraulic fracturing operations.